Why are straight lines sloping in render?
The lens shift controls on the camera exist in Blender precisely to solve this problem. Photographers use tilt-shift lenses to do the same thing.

Rather than rotating the camera upwards to the building, you can keep it pointing straight ahead, and increase the camera Y shift. This will preserve straight vertical lines.

A good visualization of that is here:
http://digitized-life.blogspot.com.es/2013/02/blender-26.html